Required Navigation Performance (RNP) standards outline the navigation performance accuracy necessary for aircraft operations within a defined airspace. Examples of RNP standards appear in Advisory Circular AC 120-29A, “Criteria for Approval of Category I and Category II Weather Minima for Approach”, published by the Federal Aviation Administration (FAA). An RNP level or type (RNP X), a value in nautical miles (NM) from the intended horizontal position of an aircraft within which the aircraft should be during at least 95 percent of its total flying time, may include performance and functional requirements. A given RNP level may provide for a total system error (TSE) allowable in lateral and longitudinal dimensions within a particular airspace. The TSE may account for path definition errors (PDE), navigation system errors (NSE), and flight technical errors (FTE). The RNP type may specify navigation requirements for the airspace or for a navigation system providing a specified level of accuracy defined by a lateral area of confined airspace within which the RNP-certified aircraft operates. For example, an RNP 0.3 level requires a normal navigational accuracy of 0.3 NM at least 95 percent of the total flight time. Similarly, an RNP 0.1 level requires a normal navigational accuracy of 0.1 NM at least 95 percent of the total flight time. Accordingly, the lower the RNP level, the greater the accuracy of the navigation system.
With the availability of satellite-based navigation systems (e.g., GPS, GLONASS), next-generation airspace systems may rely heavily on GPS-based navigation to which stringent NSE standards may be applied. An instrument approach procedure (IAP) specifying an RNP level of RNP 0.3 (or lower than the standard minima at airports not equipped with Instrument Landing Systems (ILS)) may require a method to detect GPS position errors more rapidly the 6.2 second time-to-alert standard that currently exists. A similar requirement may be made for manufacturers and/or end-users wishing to take advantage of minimum performance standards for enhanced vision systems (EVS), synthetic vision systems (SVS), combined vision systems (e.g., EVS/SVS) or enhanced flight vision systems.
One way to achieve greater navigational accuracy is to reduce the TSE. Because the TSE may comprise PDE, NSE, and FTE, a reduction of any individual component (e.g., the NSE) could reduce the TSE, producing in turn a greater level of navigational accuracy. A reduction of NSE may be achieved by monitoring and/or augmenting navigation data provided by one or more navigation systems, the systems based on independent sources of navigation data. Such independent sources could be used to validate the navigation system data by confirming that satellite-based navigation data falls within narrower desired tolerances commensurate with a lower NSE. Moreover, the confirmation of valid navigation data may permit the reduction of decision altitude (DA) or decision height (DH) of an IAP performed during an approach-for-landing flight segment, where the said IAP may be a Localizer Performance with Vertical Guidance (LPV) or any IAP with vertical guidance requiring higher data integrity.